1. Field of the Invention
The present invention relates to an image processing apparatus for forming an image by performing recording a plurality of times and a control method thereof.
2. Description of the Related Art
In recent years, many people have been printing image data processed by a personal computer (PC). Compared with the number of gradations of the image data processed by the PC, an image forming apparatus may express only a smaller number of gradations per pixel. In such a case, halftone processing is required for converting the number of gradations of input image data into the number of gradations that can be expressed by an output apparatus. As one method of the halftone processing, a dither method is known. The dither method determines an output value for each pixel by comparing a pixel value of the input image data with a threshold value corresponding to the pixel in a threshold value matrix.
Since the dither method does not require complicated operations, compared with an error diffusion method known as another method of the halftone processing, it can perform speedy processing. Further, by using a threshold value matrix of a blue noise type in which a spatial frequency of dot layout in the output image data is set to a high frequency, the dither method can generate the image data having high dispersibility equivalent to that of the error diffusion method. The Japanese Patent Application Laid-Open No. 2008-067049 discusses a method for determining, with use of the threshold value matrix of the blue noise type described above, an order of forming dots and layout of the dots when the image forming apparatus forms the image.
According to the method described in the Japanese Patent Application Laid-Open No. 2008-067049, the dither processing is performed on each recording data corresponding to record-scanning, using the threshold value matrix of a blue noise type. With this processing, the dispersibility of scanning is improved and, even if the image forming apparatus generates physical registration shift, a dot pattern having less granularity deterioration can be formed.
According to the method described in the Japanese Patent Application Laid-Open No. 2008-067049, the dot pattern of each scanning has blue-noise characteristic, however, the dot pattern in which the image data is accumulated by each scanning is not taken into consideration. Thus, when printing is performed by the image forming apparatus that forms the image by performing record-scanning on a same region a plurality of times, due to a process of forming the image, sufficient dispersibility may not be able to be acquired. As an example, a case will be described where the image is formed by four-times-record-scanning.
According to the method described in Japanese Patent Application Laid-Open No. 2008-067049, each single dot layout of the recording data corresponding to each of four-time-record-scanning has the blue-noise characteristic, and thus individually has high dispersibility. However, after first record-scanning is performed, second record-scanning is further performed, and then the accumulated dot pattern does not have the blue-noise characteristic. Therefore, in the image forming processing in which the image forming apparatus performs printing, the dots are arranged coarse and thick. Thus, liquid drops interfere each other or their interactions occur, thereby deteriorating the granularity acquired as a result.